Lithographic printing plates are known carrying a light sensitive emulsion which, after exposure, is subjected to a baking or "burning-in" process. A baking unit for baking such a lithographic printing plate following image exposure is known, for example from International patent application U.S. Pat. No. 5,166,523, European patent application EP 290245-A, German patent application DE3137430-A and WO 90/02974, comprising a housing having an inlet and an outlet for the printing plate so positioned as to define a path through the housing. A continuous surface such as a belt is positioned on one side of the path and is driven in synchronism with the plate material as it passes through the housing. The plate is urged against the continuous surface and a heat source is provided to heat the continuous surface.
Such devices can also be used for photo-thermographic processes. These are processes wherein latent images are generated by the image-wise exposure of a recording material to UV, visible or infrared radiation followed by development by uniformly heating the exposed recording material to a desired development temperature, thereby creating a visible image. Examples of photo-thermographic materials include the so-called "Dry Silver" photographic materials of the 3M company, which are reviewed by D A Morgan in "Handbook of Imaging Science", edited by A Diamond, published by Marcel Dekker Inc., New York, 1991, page 43.
Such known thermal treatment devices tend to result in non-uniform thermal treatment temperatures. Uneven heating is particularly a problem with imaging elements having a non-heat conductive base, such as a polymer material. When radiant heat is used and the plate carries images of different colours, differential heat absorption can result in non-uniform heating.